Acoustic resonators can be used to implement signal processing functions in various electronic applications. For example, some cellular phones and other communication devices use acoustic resonators to implement frequency filters for transmitted and/or received signals. Several different types of acoustic resonators can be used according to different applications, with examples including bulk acoustic wave (BAW) resonators such as thin film bulk acoustic resonators (FBARs), solidly mounted resonators (SMRs), coupled resonator filters (CRFs), stacked bulk acoustic resonators (SBARs) and double bulk acoustic resonators (DBARs).
A typical acoustic resonator comprises a layer of piezoelectric material sandwiched between two plate electrodes in a structure referred to as an acoustic stack. Where an input electrical signal is applied between the electrodes, reciprocal or inverse piezoelectric effect causes the acoustic stack to mechanically expand or contract depending on the polarization of the piezoelectric material. As the input electrical signal varies over time, expansion and contraction of the acoustic stack produces acoustic waves that propagate through the acoustic resonator in various directions and are converted into an output electrical signal by the piezoelectric effect. Some of the acoustic waves achieve resonance across the acoustic stack, with the resonant frequency being determined by factors such as the materials, dimensions, and operating conditions of the acoustic stack. These and other mechanical characteristics of the acoustic resonator determine its frequency response.
One metric used to evaluate the performance of an acoustic resonator is its electromechanical coupling coefficient (kt2), which indicates the efficiency of energy transfer between the electrodes and the piezoelectric material. Other things being equal, an acoustic resonator with higher kt2 is generally considered to have superior performance to an acoustic resonator with lower kt2. Accordingly, it is generally desirable to use acoustic resonators with higher levels of kt2 in high performance wireless applications, such as 4G and LTE applications.
The kt2 of an acoustic resonator is influenced by several factors, such as the dimensions, composition, and structural properties of the piezoelectric material and electrodes. These factors, in turn, are influenced by the materials and manufacturing processes used to produce the acoustic resonator. Consequently, in an ongoing effort to produce acoustic resonators with higher levels of kt2, researchers are seeking improved approaches to the design and manufacture of acoustic resonators.
One method that is useful in improving the kt2 of piezoelectric materials is by doping the piezoelectric material with a selected dopant, such as a rare-earth element. While doping the piezoelectric material can provide improvement in kt2, in applications in BAW resonators, other parameters may be degraded as compared to BAW resonators that do not include doped piezoelectric materials.
What is needed, therefore, is a BAW resonator that overcomes at least the shortcomings of known BAW resonators described above.